Branch Chordata 9I 



that this flesh is composed of muscle segments or 

 myotomes. Observe the jointed or segmented nature 

 of the backbone or spinal column. Its lateral bony 

 processes are analogous to the ribs of the higher verte- 

 brates. Each separate segment is a vertebra. Passing 

 along a closed tube within the spinal column is the 

 spinal cord, ending anteriorly in the brain. The two 

 (spinal cord and brain) constitute the central nervous 

 system. Nerves pass from this through openings 

 between the vertebrae to various parts of the body. 

 Break the connection between two of the vertebrae to 

 see the cavity filled by the spinal cord. Open the 

 cranium, or brain case, by removing carefully the dorsal 

 surface of the head just back of the eyes. Study the 

 lobes of the brain now exposed. Follow the continua- 

 tion of the brain into the spinal cord. Without injuring 

 the lobes of the brain, carefully dissect away sufficient 

 surrounding tissue to see the large optic nerves passing 

 to the eyes. If the lobes of the brain still remain com- 

 plete and uninjured, follow out the different parts. 

 The small median lobe extending backward is the 

 cerebellum. The pair of large rounded lobes directly 

 in front of the cerebellum, constituting the broadest 

 part of the brain, are the optic lobes. The cerebral 

 lobes, or hemispheres, are much smaller, and lie directly 

 in front of the optic lobes nearly on the median line. 

 The somewhat flattened posterior part of the brain, 

 passing from the cerebellum into the spinal cord, is 

 the medulla oblongata. Make a drawing of the brain. 



